Echo cancellers are the replacement for earlier echo suppressors that were initially developed in the 1950s to control echo caused by the long delay on satellite telecommunications circuits. Initial echo canceller theory was developed at AT&T Bell Labs in the 1960s. The concept of an echo canceller is to synthesize an estimate of the echo from the talker's signal, and subtract that synthesis from the return path instead of switching attenuation into/out of the path. This technique requires adaptive signal processing to generate a signal accurate enough to effectively cancel the echo, where the echo can differ from the original due to various kinds of degradation along the way.
Rapid advances in the implementation of digital signal processing allowed echo cancellers to be made smaller and more cost-effective. In the 1990s, echo cancellers were implemented within voice switches for the first time (in the Northern Telecom DMS-250) rather than as standalone devices. The integration of echo cancellation directly into the switch meant that echo cancellers could be reliably turned on or off on a call-by-call basis, removing the need for separate trunk groups for voice and data calls. Today's telephony technology often employs echo cancellers in small or handheld communications devices via a software voice engine, which provides cancellation of either acoustic echo or the residual echo introduced by a far-end PSTN (public switched telephone network) gateway system; such systems typically cancel echo reflections with up to 64 milliseconds delay.
Voice messaging and voice response systems which accept speech for caller input may use echo cancellation while speech prompts are played to prevent the system's own speech recognition from falsely recognizing the echoed prompts.
Examples of other systems that may include echo cancellation include: hands-free car phone systems; a standard telephone or cell phone in speakerphone or hands-free mode; standalone speakerphone, dedicated standalone “conference phones”; installed room systems which use ceiling speakers and microphones on the table; and physical coupling (vibrations of the loudspeaker transfer to the microphone via the handset casing).
In most of these cases, direct sound from the loudspeaker from the person at the far end enters the microphone almost unaltered. The difficulties in cancelling echo stem from the alteration of the original sound by the ambient space. These changes can include certain frequencies being absorbed by soft furnishings, and reflection of different frequencies at varying strength.
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.